In recent years, in a field of display devices performing image display, a display device (an organic electro luminescence (EL) display device) using a current-driven optical element as a light emitting element, the light emission luminance of which is varied based on a value of a flowing current, such as an organic EL element, has been developed and commercialization thereof is progressing. Unlike a liquid crystal element or other element, the light emitting element is a self light-emitting element, and a separate light source (a backlight) is not required. Therefore, the organic EL display device has characteristics of high visibility of an image, low power consumption, and high response speed of the element, and the like, as compared with a liquid crystal display device which requires a light source.
In such a display device, a unit pixel may include, for example, a write transistor and a drive transistor. The write transistor is adapted to select a unit pixel in which a pixel signal is to be written, and the drive transistor is adapted to supply a current to a light emitting element. For example, in PTL 1, there is disclosed a display device in which a current driving capability of the drive transistor is set lower than a current driving capability of the write transistor. Further, for example, in PTL 2, there is disclosed a display device in which a channel length L of the drive transistor is set larger than a channel length L of the write transistor. Further, for example, in PTL 3, there is disclosed a display device further including an AC transistor, and a ratio (L/W) of a channel length L to a channel width W of the drive transistor is set larger than a ratio (L/W) of a channel length L to a channel width W of the AC transistor.